1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, having a transfer device wherein a transfer sheet is adhered onto a transfer sheet bearing member (such as a transfer belt, transfer drum or the like) and a toner image formed on an image bearing member is transferred onto the transfer sheet by applying an adsorption electric field or a transfer electric field to the transfer sheet bearing member by a charger brush arranged inside of the transfer sheet bearing member.
2. Related Background Art
In image forming apparatuses such as copying machines, printers and the like, it is known that a toner image formed on an image bearing member is transferred onto a recording sheet (transfer sheet) by applying a transfer electric field to a transfer sheet bearing member (transfer belt, transfer drum or the like) constituting a transfer device by a transfer charger brush (transfer electric field applying means) of contact type arranged inside of the transfer sheet bearing member. Further, it is considered that an adsorption charger brush contacting an inner surface of a transfer sheet bearing member (transfer drum) is provided for electrostatically adhering a transfer sheet to the transfer sheet bearing member.
On the other hand, as a transfer device, generally, a transfer drum comprising a pair of opposed ring portions, a connecting portion interconnecting the ring portions, and a transfer sheet bearing film covering an opening defined by the ring portions and the connecting portion is well known.
However, in such a transfer device, as shown in FIG. 10, when conductive or semi-conductive fibers 17 and/or 27 of a transfer charger brush 15 and/or an adsorption charger brush 25 become fatigued due to a long term use thereof, as shown in FIG. 11, the fibers are bent outwardly to contact with the ring portions 5a of the transfer drum 5, with the result that, when the transfer high voltage and the adsorption high voltage from the respective electric sources (not shown) are applied to the transfer charger brush 15 and the adsorption charger brush 25, the electric currents are leaked through the ring portions 5a. Particularly, as in the illustrated transfer device, when the transfer sheet is adhered to and held by the transfer film 5f and the toner image formed on a photosensitive drum 1 is transferred onto the transfer sheet via the transfer film 5f, since the fibers 17 and 27 of the transfer charger brush 15 and the adsorption charger brush 25 are urged against the inner surface of the transfer film 5f with a relatively great force, the above problem is arisen at a relatively initial stage of the use of the transfer device, which results in a malfunction of the apparatus.
In the above-mentioned transfer device, although the adsorption charger brush 25 is provided as well as the transfer charger brush 15, even if the transfer charger brush acts also as an adsorption charger brush, the same problem will occur.